In relief printing, e.g., hand stamps, ink is transferred to a substrate by using a raised relief printing plate that is usually inked by contact with a liquid inkpad. The surface of the plate is created so that the image to be transferred appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a wooden block such that the raised surface of the printing plate can be contacted to an inkpad and the raised relief image transferred to the substrate, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
In recent years a wide variety of materials have been introduced in the market to manufacture hand stamps. There has been a notable increase in the use of photopolymerized materials to replace the traditional rubber. Other advances have simultaneously occurred in the type of mounting materials used. In particular, the use of clear acrylic mounting blocks, which enables the user to “see through the stamp” has encouraged the need for photosensitive resin compositions, which are essentially substantially transparent and able to maintain this transparency for the packaged life of the article.
In addition to the need for transparency, the hand stamp material should have sufficient softness to readily transfer the image (without distortion), have an image surface that has adequate chemical resistance towards the aqueous-based ink or alcohol-based ink that is normally used in stamp pads. It is further desired that the physical and printing properties of the hand stamp are stable and do not change during printing or storage.
Photopolymer resin compositions generally include an elastomeric binder, at least one monomer, a photoinitiator, and a free radical stabilizer. Optionally they may contain colorants or other additives to modify performance e.g., ultraviolet or humidity stabilizers, tack reducing agents, etc. Compositions capable of being converted under the influence of actinic light to solid, insoluble, tough structures are well-known and have become increasingly important in the preparation of printing plates. One of the fundamental patents relating to such compositions is U.S. Pat. No. 2,760,863 to Plambeck. In the process of the Plambeck patent, printing plates are produced directly by exposing to actinic light through an image-bearing process transparency a layer of an essentially transparent composition containing an addition polymerizable, ethylenically unsaturated monomer and an addition polymerization initiator activated by actinic light. The layer of polymerizable composition is supported on a suitable support, and exposure of the composition is continued until substantial polymerization of the composition has occurred in the exposed areas with substantially no polymerization occurring in the non-exposed areas. The unchanged material in the latter areas is removed, as by treatment with a suitable solvent in which the polymerized composition in the exposed areas is insoluble. This results in a raised relief image that corresponds to the transparent image of the transparency and that is suitable for use as a printing plate, as in letterpress and dry off-set work.
Liquid photopolymer hand stamps are generally formed by first pouring a photo-curable layer over a thin transparent plastic sheet (Cover film) placed over the image bearing surface such as a photographic negative. A second, thicker polyester transparent sheet is laminated over the resin to form the encapsulated photo-imageable material, which is sandwiched between to transparent pieces of glass equipped with sources of actinic radiation. Upon stepwise exposure to the actinic radiation, the exposed layer hardens in the exposed areas. Brushing or spraying the imaged material after removing the cover film with the aid of a suitable aqueous developer removes the unexposed areas of the photo-hardened layer leaving a printing relief which can be used for hand stamps.
Photosensitive compositions based on the reaction of polyols and diisocyanates to produce an acrylate or methacrylate terminated polyurethane oligomers diluted with various acrylate or methacrylate monomers are known. However, current photosensitive compositions used to make relief printing plates, such as hand stamps, suffer from yellowing upon exposure to ultraviolet light and sunlight.